The present invention is directed generally to electrophotographic printing, and, more particularly, to a novel photosensitive color toner that contains a combination of light-sensitive oxidizing agents, developing agents, color coupling agents, and, optionally, fixing agents.
Color photography is a well-developed process. However, while the resultant colors are reasonably stable, no process has been developed to print computer-generated color images. Even so-called xe2x80x9cinstantxe2x80x9d photography has not found use in printing computer-generated images, for example.
Briefly, in the general photographic process steps, light is directed to a silver halide (AgX) grain center. The silver halide crystal absorbs incoming energy and becomes excited (electron moves from the valence band to the conduction band). The silver halide crystal then draws an electron from the developer agent, e.g., p-phenylenediamine (PPD) and is therefore reduced (an electron is absorbed from PPD). The PPD developer is oxidized (due to the removal of the electron), leaving quinone-diimine (QDI). The QDI bonds with a color coupler or coupler ion to form an indoaniline or azomethine dye, depending upon chemistry choices. At the end of this reaction, dyes of the incident light color are created. It should be noted that each core containing color-sensitized silver halide crystals becomes colored to the wavelength it recognizes only, where the system or combination of cores then reproduces the incoming colors.
Any additional light will continue to be absorbed by AgX and continue reaction and creation of color dyes, ultimately creating black. Accordingly, the oxidation of PPD must be terminated at some point. This is accomplished by exciting the balance of unexcited AgX and saturating the AgX with electrons in a process known as xe2x80x9cfixingxe2x80x9d. Reactions are halted, and a stable colored state is the result.
Color printing has emerged as an alternative to conventional silver halide photography. The attraction of color printing is substantially immediate production of the desired image.
Presently, a number of approaches to color printing have been developed: (1) ink jet technology, such as Hewlett-Packard""s DeskJet printers, (2) dye sublimation, such as by thermal wax transfer, and (3) electrophotographic technology, such as Hewlett-Packard""s LaserJet printers.
Color ink jet technology involves the expulsion of droplets of different colors of ink comprising colorants, or chromophores, in a vehicle. The expulsion is either from a controlled series of heated resistors or from a controlled series of piezoelectric elements. However, the chromophores are not as stable, or colorfast, as one would like, and images tend to fade over time. Further, waterfastness, smearfastness, and UV-fastness problems continue to be the focus of efforts of continuing improvements. While progress is being made, work continues in these areas.
An advantage of the dye sublimation process is that the dot intensity at a given location can be varied, without having to employ different inks (as in color ink jet printing) or without the lack of variation (as in color laser jet printing). However, the resolution is fairly limited, 300 dots per inch (dpi), for example.
Color electrophotographic technology employs color toners is an alternative technology to color ink jet technology. The former tends to be more expensive initially (in terms of printer cost), but less expensive over the long term, and in any event is faster. Further, colorfastness and image fade are not major issues as they are with ink jet printing. Nevertheless, typical electrophotographic processing requires multiple process steps and complex toner mixing. Further, typically, three colors of toners are used to produce the spectrum of colors. Each toner is applied with a separate EP process. The colors to be exhibited require careful application of each toner type to provide acceptable colors, Separate toner is used to provide monochrome black.
Briefly, in the general electrophotographic process steps, a laser (or other means, such as a light emitting diode) shines energy to a finite area on the electrophotographic drum, exciting the finite area on the drum coating. The drum is coated with a photovoltaic material that retains a charge once excited. Toner is stored in a toner cartridge hopper. The toner is agitated and electrostatically charged. A field generated by voltage differences propels the charged toner particles from region to region. The toner is attracted to the laser-charged finite area on the surface of the EP drum. The toner attached to the drum rotates toward a nip. A print medium is transported through the nip. The opposing roller in the nip is charged to attract the toner towards its surface and away from the EP drum. The print medium, located in the nip between the two rollers, is the receiver of the transported toner. As the print medium leaves the nip, the toner remains in place on the surface of the print medium. Finally, the toner is fused to the print medium, typically using both heat and pressure via a roller nip. In conventional processing, the toner is a plastic material having finite melting or glass transition temperatures. Fusing is a process of melting the toner into the fibers of the print medium.
A need remains for a color toner that retains the advantages of the fore-going prior art approaches, while overcoming most, if not all, of their drawbacks. In particular, a single toner, providing all colors, including black, is desired.
In accordance with the present invention, a single color toner is provided that enables printing of all colors, including black. The color toner, which is translucent prior to exposure to light, comprises at least three layers, and, optionally, a base layer, with each layer containing separate color grain centers directed to a specific color, including color-sensitive silver halide (AgX) crystals, at least one developing agent, and color coupling chemistry. The optional base layer is employed if chemical fixing, instead of electronic fixing, is required. If used, the base layer comprises fixing agents for fixing unexcited silver halide by saturating the unreacted silver halide with electrons and for terminating developer and color coupler reactions.
Thus, in one embodiment, the color toner comprises three color layers, either employing additive color combinations (yellow, magenta, cyan) or subtractive color combinations (red, blue, green).
In a second embodiment, the color toner comprises the afore-mentioned three color layers plus the base layer of fixing agents.
Advantageously, colors can be created or modified through the application of light (selective exposure), as opposed to application of dyes, chromophores, or colored particles. Further, true black is obtainable with a single toner. Finally, image processing is mechanically and chemically simplified. Compared to color photography, white is generated simply by printing on a white print medium; no two-step process is required.
Since the color toner of the present invention includes all necessary colors that are selectively activated, only one color toner and thus only application of one color toner is required, thereby (1) reducing the application of multiple toner colors, (2) reducing the application of multiple color inks, (3) simplifying the toner processing, since selective application of xe2x80x9ccoloringxe2x80x9d, as opposed to physical application, is employed, and (4) providing flexibility in producing color or monochrome images. Advantageously, toner may be selectively applied to image locations only, and then selectively exposed.
The resolution of the resulting print is similar to that of conventional photography, namely, about 3,000 dpi. This is in contrast to (1) color ink jet (1,700 dpi), (2) color dye sublimation (300 dpi), and (3) color laser jet (1,200 dpi).